migrate std-docs.zig to std.Io

[dasimmet]

resolves #25770
possibly also #24944 if someone tests this on windows

[squeek502]

it fails to write the chunked http encoding when serving the sources inside a tar

Same problem as #24944. Presumably, sendFile is returning error.Unimplemented, and http isn't handling that fully correctly yet. Previously, that was only happening on Windows.

[dasimmet]

That change I'm not sure about myself...when the underlying writer returns error.Unimplemented we need to make sure w.end is in its original state so the writer above will re-send the header (because it does not know how much of it was written), but we need to write it before calling w.vtable.sendFile because it might flush. With this fix, the @memcopy call is wasted.
Another solution would be to follow through with sendFileReading but as the docstring of sendFile suggests, it may be less efficient than wasting the @memcopy once per file.